The present invention relates to a method of fractionating a mixture of meltable, higher molecular weight organic substances, such as, for example, fats and oils, diglycerides and triglycerides, fatty acids, and waxes and higher hydrocarbons by crystallization in the melt and subsequent separation of the crystal-containing suspension into a solid fraction and a liquid fraction, for example, a solid stearin fraction and a liquid olein fraction, and to a system for implementing the method. 2. Description of The Related Art
Prior art fractionation methods can be divided into three groups (1) crystallization in solvents, (2) crystallization and phase separation with the use of detergents (fractionation by means of a wetting agent), and (3) crystallization from the melt with mechanical phase separation without auxiliary substances (dry fractionation).
Solvent fractionation is suitable for the production of all kinds of cocoa butter substitutes and generally furnishes very good quality products. Such fractionation methods are disclosed in detail, for example, in DE 3,248,922, which corresponds to U.S. Pat. No. 4,465,703; EP 0,081,881; FR 2,427,386; which corresponds to U.S. Pat. Nos. 4,243,603 and 4,247,471; DE 2,902,235; EP 0,132,506; EP 0,189,669; EP 0,199,580; and DE 2,747,765, the disclosures of which are incorporated herein by reference.
The solvent here serves primarily to reduce the viscosity in order to increase the crystallization rate by raising the diffusion coefficient, improve selectivity by reducing foreign molecule inclusion, improve dispersibility, improve conveyability, and facilitate phase separation. In addition, the selectivity is said to be improved by interaction of the solvent with the triglycerides.
The best suited solvent is acetone which, due to its polarity, interacts with the ester groups of the fat molecules. Thus, acetone has the following advantages for the fractionation of palm oil, for example, compared to hexane. Acetone provides a higher crystallization temperature (0.degree. to -5.degree. C. instead of -20.degree. to -30.degree. C.); improved selectivity in the separation of diglycerides, which are undesirable as .beta.' stabilizers; improved selectivity in the separation of PPP; and improved selectivity in the separation of PPO. PPP and PPO are acronyms for tripalmitin and .LAMBDA.-oleo-dipalmitin, respectively.
The basic drawbacks of solvent fractionation compared to dry fractionation are the following. Investment costs are higher due to the additional apparatus required for regeneration of the solvent and for the additional fire protection devices required. Operating costs are higher due to larger quantity streams, lower crystallization temperatures and energy consumption for recovery of the solvent. Finally, there are physiological objections to solvent residues in the product.
Wetting agent fractionation generally furnishes poorer cocoa butter replacer (CBR) qualities except for the cocoa butter substitute (CBS) produced from palm kernel oil (PKO). Additionally, the process is well suited for the first stage of palm oil fractionation, i.e., the production of palm olein.
Fractionation methods employing wetting agents as solids are disclosed in, for example, DE 1,792,812 and EP 0,074,146, the disclosures of which are incorporated herein by reference. This method furnishes good results for palm kernel stearin (PKS) production. However, if used for the production of a palm mid-fraction (PMF), the results are not satisfactory since lower crystallization temperatures result in greater viscosity and poorer phase separation conditions in centrifugal separators.
Aside from the fact that no danger of fire exists and the crystallization temperatures are changed only slightly, wetting agent fractionation, compared to dry fractionation, has the same basic drawbacks as solvent fractionation.
Prior art production of cocoa butter substitutes by dry fractionation is known from FR 2,369,800, which corresponds to U.S. Pat. No. 4,205,095, the disclosure of which is incorporated herein by reference. FR 2,369,800 discloses a process for producing a cocoa butter substitute based on palm oil having an iodine number of 50-55 that is partially miscible with cocoa butter. A two-stage conventional dry fractionation is performed in stir crystallizers (crystallizers including stirring means) to produce a palm mid-fraction.
The drawbacks of the foregoing described process include the following. (1) Crystal suspensions are produced which are subjected to a mechanical solid/liquid separation at low filtration pressures. Consequently, the thus-obtained palm mid-fraction is very soft compared to palm mid-fractions obtained with the aid of solvent fractionation due to the remaining high olein percentage. (2) The use of stir crystallizers results in the crystallization being permitted to proceed only to a certain degree due to the increasing viscosity of the crystal suspension. As a consequence, the POP concentration (1,3-dipalmitoyl-2-oleoyl-glycerin) required to obtain a high quality palm mid-fraction that is completely miscible with cocoa butter cannot be realized.
CH 0,658,163, which corresponds to U.S. Pat. No. 4,594,194, the disclosure of which is incorporated herein by reference, discloses a dry fractionation method in which palm oil is used in a two-stage fractionation to yield a palm mid-fraction having an iodine number of 36-38.
The drawbacks of this method include the following. (1) It is necessary to have seed crystals in both fractionation stages and these are expensive to produce and must be present in precisely the respectively required crystal modification (.beta. modification in the first stage and .beta.' modification in the second stage). (2) Due to the particularly high demands for the required crystal modification, extremely long crystallization times of three to five days are required in the second stage.
To circumvent the above-mentioned viscosity problems when performing the crystallization to produce high quality cocoa butter substitutes in stir crystallizers, EP 0,256,760, the disclosure of which is incorporated herein by reference, proposes to perform the crystallization itself in a filter press.
The drawbacks of this manner of proceeding include the following. (1) To prevent the low viscosity starting material from escaping, before crystallization, through the olein discharge paths (filter) provided in the press, the filter surfaces must be sealed with a high melting point layer of fat before each filling process. Aside from the fact that this sealing procedure is complicated, time consuming and therefore appears to be impractical, such sealing retards the discharge of the stream of olein during the press filtration. (2) The described required total periods of dwell of the material within the press far exceed the customarily employed pressing duration of approximately one hour. This increases the space requirement within the press for a given throughput. Consequently, particularly high investment costs are to be expected in view of the high compressive pressures of about 30 bar that are needed.
Also, in order to circumvent the above-described problems at high degrees of crystallization, GB 2,220,672, the disclosure of which is incorporated herein by reference, discloses a dry fractionation process for low lauric and non-lauric fats. This process is composed of the steps of (1) static crystallization in flat containers without a stirring device; (2) comminution of the hardened crystal cake in such a way that it is converted into a pumpable consistency; and (3) press filtration in order to separate the crystal cake into a crystalline stearin fraction and a liquid olein fraction at pressures of up to 28 bar.
The process disclosed in GB 2,220,672 for producing and processing crystal suspensions with high crystal concentrations which are subjected to a solid/liquid separation under increased pressures, had already been described by Wong Soon in "A Development Approach to Cocoa Butter and Cocoa Butter Replacers" Kuala Lumpur, 1987, for the processing of lauric fats.
However, GB 2,220,672 mentions explicitly that the disclosed method is not suitable for the processing of lauric fats since lauric fats form crystal suspensions which, after process step (2), cannot be converted into a pumpable consistency by comminution.
Further drawbacks of the GB 2,220,672 process include the following. Experience has shown that static crystallization is identifiably characterized by an undesirable inhomogeneity of the crystals formed due to the fact that the developing higher density crystals sink down and are compacted at the bottom of the vessel. This reduces the crystal surface available for the attachment of further molecules from the melt. Consequently, substance transport is impaired which leads at least to longer crystallization times and, in the most unfavorable cases, even to undesired incorporation in the crystals of triglyceride compositions due to kinetic effects.
This shall be explained with the following example. In the manufacture of particularly high quality palm mid-fractions that are completely miscible with cocoa butter, it is important to crystallize the highest possible percentages of POP (1,3-dipalmitoyl-2-oleoyl-glycerin) to obtain a melting behavior similar to cocoa butter. The POO molecules (1-palmitoyl-2,3-dioleoyl-glycerin) also contained in palm oil fractions are undesirable in the crystal since they lead to softening of the material. Compared to the crystallization of POP molecules, the crystallization of POO molecules is kinetically inhibited, that is, POO molecules have a slower attachment rate to existing crystals than POP because of steric impediments caused by a larger number of double bonds in the molecule.
According to the above, the very long crystallization times mentioned in GB 2,220,672 (25 to 30 hours) lead to undesirably high POO percentages relative to POP.
In GB 2,220,672, air at a constant temperature of 10.degree. to 15.degree. C. is preferably employed as the coolant for the crystallization of palm olein. Thus, in the so-called linear cooling period between 35.degree. and 20.degree. C. mentioned there, cooling rates of 6.degree. to 120.degree. C. per hour result. These comparatively high cooling rates occur during the sensitive nucleation phase and, as experience has shown, lead to crystals that include a larger percentage of undesirable .beta.' or .alpha. modifications.
In addition, the high temperature differences occurring at the beginning of the crystallization phase cause the material to be relatively heavily under-cooled so that the oil temperature changes drastically at the beginning of the crystal formation phase. This again results in renewed excessive temperature differences during the onset of crystallization, again leading to crystals of undesirable modification.
Moreover, experience has shown that excessive temperature differences at the beginning of a static crystallization process lead to crust formations at the phase interfaces which impede heat transfer. This again results in longer crystallization times so that the already described drawbacks are augmented.
Another drawback that should be given particular emphasis is that the procedure according to GB 2,220,672 results in crystals which are able to only withstand compressive pressures of up to a maximum of 28 bar. As experience has shown, such compressive pressures are in no way sufficient for the production of particularly high quality cocoa butter substitutes that have steep melting curves similar to cocoa butter. It is here important, in principle, to enrich a certain triglyceride class having a similar melting point (about 32.degree. C. to 35.degree. C.) to such an extent, e.g., POP in the palm mid-fraction or LLL (trilaurin) for palm kernel oil. The mechanical solid/liquid separation is thus the better the more liquid is pressed out of the spaces between the crystals.
Thus, it is an object of the present invention to avoid the drawbacks occurring in the processes described above and to produce fractions from mixtures of economical raw materials, such as, for example, fat fractions from palm oil, palm kernel oil and others, which, due to their physical and chemical characteristics, are able to replace relatively expensive substances, for example, cocoa butter.
It is another object of the present invention to avoid dilution by auxiliary substances of the mixture to be fractionated, such as dilution by solvents or wetting agents.
It is yet another object of the present invention to make it possible to realize a favorable fractionation of the most varied mixtures of meltable, higher molecular weight organic substances, particularly if during crystallization the resulting solids content is so high that the suspension solidifies.